The invention relates to electronic devices, and, more particularly, to integrated circuit insulation materials and fabrication methods.
The performance of high density integrated circuits is dominated by metal interconnect level RC delays due to the resistivity of the metal lines and the capacitive coupling between adjacent lines. The capacitive coupling can be reduced by decreasing the relative permittivity (dielectric constant) of the dielectric (insulator) between adjacent lines.
Various dielectric materials have been suggested for use in silicon integrated circuits; namely, silicon dioxide (currently the dominant dielectric material with a relative permittivity about 4.0), inorganic materials such as flourinated silicon dioxide (relative permittivities about 3.0-4.0), organic materials such as polyimide, parylene, amorphous teflon (relative permittivities about 1.9-3.9), and porous dielectrics such as silicon dioxide xerogels (relative permittivity depending upon pore size and density, typically 1.3-3.0). Indeed, the pore sizes in silica xerogels are usually much smaller than the integrated circuit feature size; see Smith et al., Preparation of Low-Density Xerogel at Ambient Pressure for Low k Dielectrics, 381 Mat. Res. Soc. Symp. Proc. 261 (1995). The porosity can be up to 99%.
Thin film silica xerogels for integrated circuit dielectrics can be fabricated by the generic steps of (1) precursor preparation, (2) spin coating, (3) aging, (4) solvent exchange, and (5) drying. The acid-base sol-gel reactions could be as follows:
Hydrolyze an alkoxide in a solvent: ##STR1## the solvent conveniently could be ethanol.
Then condensation: ##STR2## The condensation would be controlled so that spin coating occurs after partial condensation to an appropriate viscosity.
The solvent exchange step replaces the original solvent residing within the pores of the condensed network by low-surface-tension solvent to reduce the capillary pressure during drying and minimize the collapse of pores. U.S. Pat. No. 5,561,318 discloses variations of the process.
However, porous silica has not yet become manufacturable.
Shea et al., Arylsilsesquioxane Gels are related Materials, New Hybrids of Organic and Inorganic Networks, 114 J. Am. Chem. Soc. 6700 (1992), describe gels made from hydrolysis and condensation of monomers such as benzene with two ethoxysilyl groups; the gels were dried and the macroporous polymer network collapsed yielding a microporous polysilsesquioxane xerogel.